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Home , Bystropogon, Clinopodium, Conradina, Conradina brevifolia, Conradina canescens, Conradina etonia

Review The (): A Review

Noura S. Dosoky 1 and William N. Setzer 1,2,*

1 Aromatic Research Center, 615 St. George Square Court, Suite 300, Winston-Salem, NC 27103, USA; [email protected] 2 Department of Chemistry, University of Alabama in Huntsville, Huntsville, AL 35899, USA * Correspondence: [email protected]; Tel.: +1-256-824-6519

Received: 24 February 2018; Accepted: 10 March 2018; Published: 11 March 2018

Abstract: Conradina (Lamiaceae) is a small genus of native United States (US) limited to Florida, Alabama, Mississippi, Tennessee and Kentucky. Three species of Conradina are federally listed as endangered and one is threatened while two are candidates for listing as endangered. The purpose of the present review is to highlight the recent advances in current knowledge on Conradina species and to compile reports of chemical constituents that characterize and differentiate between Conradina species.

Keywords: Conradina; essential oil; ursolic acid; cytotoxicity; antimicrobial; antileishmanial; phylogenetic analysis

1. Introduction Conradina A. Gray (Lamiaceae) is a small genus of morphologically distinctive, narrow-leaved, minty-aromatic endemic to the southeastern United States of America (USA) [1]. It consists of six to nine US native species. According to database [2] and Integrated Taxonomic Information Species (ITIS), the acceptable number of species for the genus is only six ( A. Gray, C. cygniflora C.E. Edwards, Judd, Ionta & Herring, C. etonia Kral & McCartney, C. glabra Shinners, C. grandiflora Small and C. verticillata Jennison) while C. brevifolia Shinners, C. montana Small and C. puberula Small are considered synonyms of other species. The Missouri Botanical Garden, however, lists nine distinct species [3]. Conradina species are characterized by very dense hairs on their lower leaf surfaces and by a sharply bent corolla tube in the flowers [1,4]. Asa Gray established the genus Conradina in 1870, named for the American botanist Solomon White Conrad [5]. Conradina species grow well in xeric with well-drained sandy . It is thought that Conradina may be a pioneer species in disturbed areas since it has the ability to colonize xeric disturbed habitats [1,5]. Each species of Conradina occupies a separate geographical region [1,6]. Five of the species are endemic to Florida, one is native to west Florida, south Alabama, and south Mississippi, and one is endemic to north-central Tennessee and Kentucky (Figure1). Due to destruction from residential, commercial, and agricultural land conversions and their very restricted distribution, three of these species are on the United States Fish and Wildlife Service (USFWS) federal list as endangered (, C. glabra, and C. etonia)[5], one as threatened (C. verticillata)[6] and two species are considered candidates for listing as endangered (C. grandiflora and C. cygniflora). The aim of the present review is to summarize the literature in order to document secondary metabolites that characterize and differentiate between Conradina species. We shall also assess the reported biological activities of Conradina species, with particular focus on their phylogenetic affinities and anti-microbial, anti-leishmanial and cytotoxic properties.

Plants 2018, 7, 19; doi:10.3390/plants7010019 www.mdpi.com/journal/plants Plants 2018, 7, 19 2 of 10 Plants 2018, 7, 19 2 of 10

FigureFigure 1. 1.Locations Locations of of known known populations populations of ofConradina Conradinaspecies. species.

2.2. Description Description ConradinaConradinarepresents represents a a group group of of evergreen, evergreen, compact, compact, perennial perennial shrubs shrubs with with virgate virgate branches. branches. LeavesLeaves are are aromatic aromatic and and needle-likeneedle-like withwith narrow,narrow, entire, revolute leaf-blades. Flowers Flowers are are solitary solitary or orin in a agroup group of of few few in inaxillary axillary cymes. cymes. The The calyx calyx is two-lipped; is two-lipped; the thelower lower lip has lip hastwo twolong, long, narrow narrow lobes lobeswhile while the upper the upper lip has lip hasthree three short short broad broad lobes. lobes. Corolla Corolla is white is white to purple to purple in color, in color, two-lipped; two-lipped; the thelower lower lip lip has has three three lobes lobes and and usually usually dotted dotted whil whilee the the upper upper lip lip is iserect erect and and slightly slightly concave. concave. ConradinaConradina canescens canescensA. A. Gray Gray (false (false ) rosemary) is is the the only only species species of of this this genus genus that that is is relatively relatively commoncommon in in its its range range and and the the most most morphologically morphologically variable variable (Table (Table1) 1). It. It is is native native to to a a small small area area of of westwest Florida Florida and and south south Alabama Alabama and and southern southern parts parts of of Mississippi Mississippi [ 7[7].]. It It occurs occurs on on sunny, sunny, dry dry sand sand soilssoils on on coastal coastal dunes, dunes, inin sandsand scrubsscrubs and in dry longleaf longleaf pineland pineland ecosystems. ecosystems. It Itshows shows tolerance tolerance for forthe the heat heat and and humidity humidity of of the the Southeast Southeast and and is consideredconsidered aa drought-tolerantdrought-tolerant landscapelandscape plant plant that that growsgrows best best in in lean, lean, sandy sandy soils soils [8]. [8].C. canescensC. canescensis one is one of the of indicatorthe indicator species species for wooded for wooded dunes dunes [9] and [9] isand an excellentis an excellent candidate candidate for restoration for restoration of coastal of areas.coastal It areas. can be It used can be for used beach for projects beach thatprojects require that plantingrequire onplanting the back on sidethe back of primary side of dunes,primary or dunes, any side or ofany secondary side of secondary dunes. It hasdunes. also It been has also used been in gardensused in asgardens a groundcover as a groundcover as the branches as the branches can reach can up reach to 1 m up in to spread. 1 m in Falsespread. rosemary False rosemary was easily was propagatedeasily propagated in the University in the University of Florida of InstituteFlorida Institute of Food of and Food Agricultural and Agricultural Sciences Sciences Extension Extension using softwoodusing softwood stem cuttings stem cuttings from terminal from terminal shoots taken shoots during taken theduring growing the growing season. season. Morphologically,Morphologically,C. C. brevifolia brevifoliaShinners Shinners (short-leaved (short-leaved rosemary) rosemary) is is very very similar similar to toC. C. canescens canescens, , differingdiffering by by its its shorter shorter leaves leaves and and greater greater number number of of flowers flowers per per axil, axil, and and it it is is also also similar similar to to the the endangeredendangeredC. C. glabra glabra[ 1[1,5].,5]. C.C. brevifoliabrevifolia waswas describeddescribed asas aa newnew speciesspecies byby Shinners Shinners [ 4[4].]. Many Many taxonomictaxonomic reviews reviews of ofConradina Conradinahave have upheld upheldC. C. brevifolia brevifoliaas as a a distinct distinct species species yet yet it it is is taxonomically taxonomically questionablequestionable[1 ,[1,6,10,11].6,10,11]. Some Some taxonomists taxonomists are stillare uncertainstill uncertain that C. that brevifolia C. brevifoliais a true is different a true different species andspecies therefore and therefore they treat they it as treat a it as a synonym to C. canescens. to C. canescens. C. brevifolia C. brevifoliahas a very has restricted a very restricted range in range the middlein the ofmiddle the Lake of the Wales Lake Ridge Wales in Central Ridge Floridain Central [12 –Florida14] and [12–14] has been and listed has as been a federally listed as endangered a federally speciesendangered in 1993 species [5]. Habitat in 1993 destruction [5]. Habitat for destruction agricultural for and agricultural residential and purposes residential is the mainpurposes threat is tothe thismain species. threat to this species. C.C. etonia etoniaKral Kral & & McCartney McCartney (Etonia (Etonia rosemary) rosemary) was was discovered discovered in in the the Etonia Etonia Scrub Scrub in in Putnam Putnam County,County, Florida Florida in in September September 1990 1990 and and describeddescribed asas aa newnew speciesspecies in 1991 [6]. [6]. C.C. etonia etonia isis similar similar to toC.C. grandiflora grandiflora inin general general appearance appearance and and the the flowers flowers are are large large and and quite quite similar similar but but the the leaves leaves of of C. C.etonia etonia areare broader broader than than C.C. grandiflora grandiflora and have clearly visible laterallateral veinsveins onon thethe lowerlower surface surface [ 1[1,15].,15]. C. etonia is found in very limited areas of deep white sand scrub dominated by sand pines () and scrubby (Quercus spp.) on dry soils. Its entire known range is within a subdivision

Plants 2018, 7, 19 3 of 10

C. etonia is found in very limited areas of deep white sand scrub dominated by sand pines (Pinus clausa) and scrubby oaks (Quercus spp.) on dry soils. Its entire known range is within a subdivision of Putnam County containing streets and a few residences, which make it the most narrowly distributed species of Conradina [6]. C. etonia was listed as a federally endangered species in July 1993 [5]. The latest survey of Etonia rosemary counted about eight subpopulations on Etonia Creek State Forest containing fewer than 1000 total individuals [16]. The major threats facing C. etonia include habitat loss due to development projects for residential housing, horticultural collection, hurricane damage and invasive species or lack of natural processes, such as fire [17,18]. Because C. etonia requires light but can tolerate the light shade of openings the in scrub, fire suppression is one of the major limiting factors as it results in over-shading by overstory scrub vegetation rendering the habitat less favorable for C. etonia [18]. C. grandiflora Small (large-flowered rosemary) is endemic to the scrub habitat of Florida’s east coast between Daytona Beach and Miami as well as near Orlando and Okeechobee County [5]. A regime of frequent fires benefits C. grandiflora because it cannot tolerate shade and requires an open sunny habitat. C. grandiflora is a candidate for listing as an endangered species but of a lower priority than other Conradina species [5]. Loss of the habitat is the major threat to this species. C. glabra Shinners (Apalachicola rosemary) was described in a sand hill community east of the Apalachicola River in Liberty County, Florida in 1962 [1,4]. This species is very narrowly distributed and restricted to Liberty County, Florida. Out of its seven known locations, six C. glabra subpopulations (artificially divided by the Florida gas transmission pipeline) are remaining, all of which are located on private silvicultural lands and subject to much disturbance. Its other historical locations were converted to pine plantations, and can no longer support this species. Because of homozygosity and inbreeding, many of flowers of C. glabra are male sterile, and the stamens are very malformed [1]. It was added to the federal list of endangered species in 1993 [5,19,20], primarily due to habitat loss and modification of forestry practices. There is no record of how abundant C. glabra was before its habitat was altered because the silviculture operations began before its discovery. C. verticillata Jennison (Cumberland rosemary) is the only species of this genus that is not found in Florida. The species was established in 1933 [21]. It is a rare species, existing only as three small subpopulations in Tennessee and one subpopulation in Kentucky [22]. C. verticillata is found only in boulder bars, cobble bars, sand bars and gravel bars in close proximity to rivers on the Cumberland Plateau of north-central Tennessee and southeastern Kentucky [22]. The preferred habitat conditions include open to slightly-shaded area, moderately deep well-drained sandy , periodic flooding, and topographic features like narrow channels or depressions on gravel bars. C. verticillata is the only species that is triploid (n = 3) and accordingly has a lower ability to reproduce and disperse sexually and greatly reduced seed germination rates. The fresh seeds of C. verticillata are physiologically dormant and require cold stratification to germinate [23]. It has been on the federal list of threatened species since 1991 [24]. The major threats facing C. verticillata are habitat destruction, general deterioration of water quality, competition and shading by woody plants. In 2009, analysis of patterns of genetic structure based on microsatellites in Conradina led to the identification of C. cygniflora Edwards, Judd, Ionta & Herring [25]. C. cygniflora can only be found in Dunns Creek state park in south-central Putnam County, Florida. It occupies nine tightly-clustered sites that probably form around two to four self-sustaining subpopulations. C. cygniflora carries several unique morphological characters that distinguish this species from all other described Conradina species including thin-walled unicellular hairs, epidermis features, and larger calyces. Due to its exceptionally limited geographic distribution and low number of individuals, C. cygniflora is considered a candidate for listing as federally endangered [25]. There is very little information in the literature about C. montana and C. puberula. C. montana Small was reported in sandy woods and ravines, Appalachian plateau near Rugby, Tennessee while C. puberula Small was observed in pine-lands, northern Gulf coast region, Florida [26,27]. Some taxonomic discrepancies surround C. montana and C. puberula since some experts treat both species as synonyms of C. verticillata and C. canescens, respectively. Plants 2018, 7, 19 4 of 10

Table 1. Morphological characteristics of Conradina species.

Species Morphological Characteristics References Small , up to 1 m high. Leaves are 7 to 20 mm long, mostly longer than the internodes. Leaf blades are pubescent on both sides. One to three flowers per axil, C. canescens [5] lower corolla-lip 8–10-mm long; lateral lobes longer than wide. Calyx-tube hirsute or villiou-hirsute. Small shrub, up to 1 m high. Leaves are short fleshy 6.0 to 8.2 mm long, mostly C. brevifolia a shorter than the internodes, covered with short downy hairs and many tiny glands [5,11] on the upper side. One to six lavender flowers per axil. Straight slender shrub, about 1.5 m high. Leaves have hairy, veiny, glandular C. etonia blades 1.5–3 cm long and 3–9 mm wide with tightly rolled edges. Three to seven [5,15,18] flowers per axil. Pink to lavender in color with darker dotted lower . Small shrub, about 80 cm high but some individuals reach up to 2 m. Leaves are opposite, up to 1.5 cm long, hairless on the upper surface. Two to three flowers per C. glabra [5,19,20] axil. Corolla is 1.5–2 cm long, white to pale lavender in color with a band of purple dots on the lower lip. Erect shrub, 1.5–2.0 m high, with hairy branches and twigs. Leaves are hairy, glandular, up to 1.5 cm long. Year-round hairy lavender flowers with darker C. grandiflora [5] lavender spots, lower lip is 12–15 mm long with lateral lobes longer than wide. This species has the largest flowers of genus Conradina. Erect shrub, 0.5 m high with reclining branches. Leaves are about 2.5 cm long, very narrow, and arranged in tight bunches that appear as whorls around the stems. C. verticillata [24] Flowers are 2.5 cm long, purple to white and borne in leaf-like clusters of bracts at the ends of the stems. Short shrub less than 0.5 m high with diffuse branches. Leaves are narrowly linear, C. montana b 5–16 mm long. Leaf blades are glabrous on the upper surface. Minute flowers with [23] corolla 3.5–4 mm long. Calyx-tube hirsutulous. Short shrub of about 0.5 m high with numerous slender branches. Leaves are narrowly linear but strongly revolute and clavate, 12–20 mm long. Calyx-tube C. puberula a [26,27] minutely canescent, 5–7 mm long. Flowers appear in racemes of 2–6 per axil, with corolla 4–5 mm long. Virgate shrub up to 1 m high, branches are erect to spreading, internodes 5–43 mm long. Leaves persistent, appearing fascicled- verticillate; narrowly obovate, 9–33 mm long. The abaxial leaf surface is densely-covered by simple unicellular hairs. Cymes carry 1–5 subsessile flowers, densely pubescent, 1.3–12.5 mm long. C. cygniflora [25] Large calyx of 8.5–11 mm long; densely covered with simple hairs, upper lip upcurved, 3.6–4.4 mm long, lower lip 4.3–5.5 mm long. Corolla strongly bilabiate, 20–29 mm long, lavender, shading to white in throat, with purple spots; abaxial surface of upper lip darker lavender. a C. brevifolia and C. puberula are sometimes treated as synonyms to C. canescens, b C. montana is sometimes thought as a synonym to C. verticillata.

3. Phylogenetic Studies Conradina is a member of Lamiaceae and belongs to a clade of New World . The closely related genera that share similar morphology and habitat preference with Conradina include spp., spp., spp., and the woody, southeastern U.S. species of such as C. ashei, C. georgianum, C. coccineum, and C. dentatum [28]. Despite the fact that Conradina species are morphologically distinguishable, there are two problems about species status based on morphology. First, C. brevifolia (endangered species) shares a lot of morphological characteristics with C. canescens (relatively widespread in its range). Second, populations of Conradina in Santa Rosa County, Florida, have morphological characteristics in common with both C. glabra (endangered species) and C. canescens. Due to the endangered status of four Conradina species, it is important to resolve the species relationships, define species limits and elucidate the evolutionary processes in order to help with conservation plans. Conradina was subjected to Plants 2018, 7, 19 5 of 10 several phylogenetic analyses using four data partitions: plastid DNA, internal transcribed spacer (ITS), and two members of the GapC gene [29–31]. These analyses did not resolve species relationships in Conradina. In agreement morphological evidence, ITS results supported a monophyletic Conradina while plastid DNA results did not. Some Conradina populations showed moderate levels of inbreeding, but inbreeding does not seem as a major factor. Because previous analyses failed to resolve species relationships separately, combined analyses were carried out but the results still did not resolve the relationships within Conradina. Studies using genotype data from quickly evolving microsatellite loci suggested that both ancient interspecific hybridization and incomplete lineage sorting of ancestral polymorphism have likely occurred in Conradina and that based on the patterns of genetic structure (which corresponds to species bounderies), Conradina species have diverged genetically from one another (nonmonophyly) despite having unique distribution patterns and distinguishing morphological characteristics [31].

4. Essential Oils and Non-Volatile Components

4.1. Essential Oils The essential oils obtained from the aerial parts of Conradina spp. are rich in terpenes, terpenic aldehydes and ketones, and terpenic alcohols [32,33]. Terpenes released from Conradina are allelopathic, and are believed to help prevent wildfires [34,35] and to protect against insects [32]. The main components quantified in the essential oils of Conradina species with the respective percentages (%) are summarized in Table2. The essential oil of C. cygniflora has not been studied yet. 1,8-Cineole and camphor are the major components of C. canescens, C. brevifolia, C. glabra and C. verticillata [7]. The common components found in all Conradina species (excluding C. cygniflora) share a terpenoid skeleton but vary in cyclization and oxygenation (Figure2). These common components include 1,8-cineole (bicyclic monoterpene ether), camphor (bicyclic monoterpene ketone), α-pinene (bicyclic monoterpene), β-pinene (bicyclic monoterpene), β-myrcene (acyclic monoterpene), borneol (bicyclic monoterpene alcohol), sabinene (bicyclic monoterpene), camphene (bicyclic monoterpene), and β-caryophyllene (bicyclic sesquiterpene).

Table 2. Chemical composition of essential oils of Conradina species.

Species Major Oil Components (%) Unique Component(s) Reference Camphor (9.7–17.54%) and 1,8-cineole C. brevifolia a α- and β-farnesene [7] (1.97–4.86%) Camphor (0.27–23.64%), 1,8-cineole (0.17–3.34%), C. canescens a none [7] cis-pinocamphone (0–8.74%) 1,8-cineole (5.2–25.2%), camphor (5.7–8.0%), α-pinene (3.2–5.6%), p-cymene (3.3–5.9%), C. canescens b cis-pinocamphone (1.3–5.5%), myrtenal -[33] (5.2–8.1%), myrtenol (3.4–9.2%), verbenone (4–4.5%), and myrtenyl acetate (5.0–5.4%) Camphor (30.55–35.65%), limonene (3.77–6.33%), β-elemene, 4-carene and C. etonia a camphene (2.92–3.75%), and β-caryophyllene [32] α-terpineol (2.95–6.54%) 1,8-cineole (2.38–7.34%) and camphor C. glabra a Dolcymene and bornyl acetate [7] (11.78–15.88%) β-pinene (4.38–5.81%) and β-cubebene C. grandiflora a Calarene and β-pinone [7] (1.95–6.56%) 1,8-cineole (3.15–3.78%) and camphor Germacrene B and C. verticillata a [7] (5.81–8.35%) 2,5,6-trimethyl-1,3,6-heptatriene C. cygniflora Data not available N/A N/A a Essential oil obtained by solvent extraction, b essential oil obtained by hydrodistillation. Plants 2018,, 7,, 19 6 of 10 Plants 2018, 7, 19 6 of 10

O O O OH α β 1,8-Cineole CamphorO -Pinene -Pinene BorneolOH 1,8-Cineole Camphor α-Pinene β-Pinene Borneol

HH HH Sabinene Camphene β-Caryophyllene β-Myrcene Sabinene Camphene β-Caryophyllene β-Myrcene Figure 2. Chemical structures of common components found in six species of Conradina . FigureFigure 2. ChemicalChemical structures structures of common components found in six species of Conradina. 4.2. Nonvolatile Components 4.2. Nonvolatile Nonvolatile Components Conradina is a reservoir of several important bioactive molecules. However, the chemical studiesConradina on Conradina isis aa reservoir reservoir are very of severalof limitedseveral important importantin number. bioactive bioactive Six molecules. compounds molecules. However, were However, separated the chemical the fromchemical studies the studiesonchloroformConradina on extractConradinaare very of limited C.are canescens very in number. limited leaves Six (Figurein compounds number. 3): ursolic Six were compounds separatedacid, betulin, from were β the-amyrin, separated chloroform myrtenic from extract acid, the of chloroformC.n-tetracosane, canescens leavesextract and (Figure oleanolicof C. 3canescens): ursolicacid [36]. leaves acid, C. betulin, canescens(Figureβ 3):-amyrin, leaves ursolic are myrtenic acid,considered betulin, acid, an -tetracosane,βnatural-amyrin, source myrtenic and for oleanolic ursolic acid, nacid-tetracosane, [[37].36]. C. canescens and oleanolicleaves areacid considered [36]. C. canescens a natural leaves source are for considered ursolic acid a natural [37]. source for ursolic acid [37].

H H OH H H CO H H CO H OH 2 2 H H CO H H CO H H H 2 H 2 HO H HO H HO H H H H HO HO HO H Betulin HUrsolic acid OleanolicH acid Betulin Ursolic acid Oleanolic acid

CO2H H CO2H H H n-Tetracosane HO H Myrtenic acid H HO Myrtenic acid n-Tetracosane βH-Amyrin β-Amyrin Figure 3. Chemical structures of nonvol nonvolatileatile components isolated from Conradina canescens.. Figure 3. Chemical structures of nonvolatile components isolated from Conradina canescens. 5. Biological Activities 5. Biological Activities 5.1. Allelopathic Activity 5.1. Allelopathic Activity The termterm allelopathy allelopathy describes describes the chemicalthe chemical interactions interactions between between plants whereplants one where plant one interferes plant withinterferesThe the germination termwith theallelopathy germination and growth describes and of growth another the chemical of plant. another Allelopathic interactions plant. Allelopathic compounds between compounds plants are thought where are to thoughtone enter plant the to interferessurroundingenter the withsurrounding environment the germination environment via volatilization,and growth via volatilizati of leachinganotheron, plant. with leaching rain,Allelopathic andwith decomposition rain, compounds and decomposition are of plantthought litter, toof entertherebyplant thelitter, inhibiting surrounding thereby the inhibiting growth environment of the competitors growth via volatilizati of or comp speciesetitorson, that leaching or may species threaten with that rain, the may plant’sand threaten decomposition survival. the plant’s In this of plantmanner,survival. litter, theIn thereby plantthis manner, may inhibiting have the an plant the ecological growthmay have role of incompan its ecological ecosystemetitors or role species by in affecting its that ecosystem plantmay threaten spacing, by affecting the succession, plant’s plant survival.andspacing, community succession,In this compositionmanner, and communitythe [plant35]. Terpenoids, may composition have an especially ecological [35]. Terpenoids, monoterpenoids, role in its especially ecosystem are consideredmonoterpenoids, by affecting one ofplant theare spacing,14considered allelochemical succession, one of classes the and 14 [allelochemcommunity38,39] thatical play composition classes an important [38,39] [35]. that roleTerpenoids, play in the an interactions important especially role between monoterpenoids, in the plants interactions [40 ,are41]. consideredC.between canescens plants oneis thought of[40,41]. the 14 to C. allelochem play canescens a significant icalis thought classes ecological [38,39]to play rolethat a significant inplay maintaining an important ecological a healthy role role in in scrub the maintaining interactions ecosystem a betweenbyhealthy inhibiting scrub plants theecosystem [40,41]. germination C. by canescens inhibiting of native is thought the grasses germinatio to [42 play]. Itn wasa of significant native suggested grasses ecological that [42]. by inhibiting Itrole was in suggestedmaintaining germination that a healthyby inhibiting scrub germination ecosystem by of inhibitingthe grasses, the the germinatio developmentn of ofnative the more grasses fire-prone [42]. It wassand suggested hill ecosystem that by inhibiting germination of the grasses, the development of the more fire-prone sand hill ecosystem

Plants 2018, 7, 19 7 of 10 of the grasses, the development of the more fire-prone sand hill ecosystem is prevented, and the scrub ecosystem is maintained [35]. Because detailed knowledge of allelopathic actions in natural plant communities can provide excellent models for new strategies in developing highly selective herbicides, the allelopathic effect of C. canescens has been a subject of several studies [34,35,43]. Water washes of fresh C. canescens leaves were reported to have strong inhibitory effects on germination and growth of Schizachyrium scoparium [34,43]. The essential oil of C. canescens and purified ursolic acid remarkably inhibited the germination of Lactuca sativa and Lolium perenne [37]. The phytotoxic activity of C. canescens oil can be attributed to the high concentration of 1,8-cineole which can inhibit the seed germination and seedling growth of lettuce in a dose-dependent manner [44–48] by strongly inhibiting mitochondrial respiration and all stages of mitosis [49]. The presence of significant quantities of ursolic acid suggests that this compound might contribute to the strong allelopathic effect of C. canescens [50,51]. Ursolic acid is thought to act as a natural detergent by leading water-insoluble monoterpenes to form micelles, rendering them water-soluble, thereby enhancing their ability to leach into rainwater for delivery into the soil [42,50]. Ursolic acid helps to co-solubilize the allelopathic monoterpenes in water to be more effective [52].

5.2. Antibacterial and Antifungal Activity The essential oil of C. canescens has no antibacterial activity but has a slight antifungal activity against Botrytis cinerea [33] while the crude extracts showed good antifungal activity against Botrytis cinerea [36]. Ursolic acid, the major component in C. canescens leaves was reported to have selective antibacterial activity against S. aureus [36,53] while its isomer, oleanolic acid, was active against S. aureus and Pseudomonas aeruginosa. Interestingly, isolated n-tetracosane showed antibacterial activity against Staphylococcus aureus and S. epidermidis and antifungal activity against Aspergillus niger and B. cinerea. Isolated myrtenic acid was active against S. aureus, A. niger, B. cinerea and Candida albicans [36].

5.3. Cytotoxic Activity The essential oil of C. canescens has no significant in vitro cytotoxic activity against the human breast tumor cell lines MCF-7 and MDA-MB-231 [33]. The crude chloroform extract of C. canescens showed a significant cytotoxic activity against MCF-7 (human breast tumor), MDA-MB-231 (human breast tumor) and 5637 (human bladder tumor) cell lines which is attributed to the presence of betulin and ursolic acid, which when tested individually showed strong effects against all tested cell lines [36].

5.4. Antileishmanial Activity The crude extracts have promising antileishmanial activity against promastigotes and intracellular amastigotes of Leishmania amazonensis. However, the dichloromethane extract had some cytotoxicity on the host cells. The antileishmanial activity of the nonpolar extract was attributed to the presence of ursolic acid and betulin being the major constituents [36].

6. Conclusions This review summarizes the current status, distribution, chemical value and biological studies on genus Conradina. There is still some debate about the exact number of Conradina species as well as the classification/ of this genus and more studies, especially genetic studies, are still needed. The available information increases the importance of protecting Conradina from extinction. Future studies on this genus may provide valuable information on the mechanism of evolution.

Acknowledgments: This work was carried out as part of the activities of the Aromatic Plant Research Center (APRC, https://aromaticplant.org/). The authors are grateful to doTERRA¯ International (https://www.doterra. com/US/en) for financial support of the APRC. Author Contributions: N.S.D conceived, collected the references, and wrote the initial draft of the review; W.N.S. edited the review. Plants 2018, 7, 19 8 of 10

Conflicts of Interest: The authors declare no conflicts of interest.

References

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